The influence of water vapor on bentonites or smectites is of interest in many different fields of applied mineralogy such as nuclear-waste sealing or casting in the foundry industry. The water vapor affects the smectite surface and perhaps its structure probably leading to mostly unfavorable changes in its properties. In this first part of the present study, the influence of hot water vapor (200°C) on the physicochemical and mineralogical properties of smectite-group minerals was studied. After the steam treatment, turbidity measurements, methylene-blue sorption, water adsorption, and cation exchange capacity (CEC) were measured on both untreated and treated samples. Mineralogical changes were monitored by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) was used to measure O, Al, and Si. Only a few parameters showed differences between the untreated and vapor-treated samples. Sedimentation volumes (SV) decreased following the treatment. As shown by XRD and XPS, the crystalline structure of smectite remained unaffected by the steam treatment. Equivalent sphere diameters (ESD) were not affected systematically by the steam treatment. Differences in CEC values between untreated and treated samples were observed, but only for smectites with monovalent interlayer cations. From the variety of different measurements the conclusion of the present study was that steam treatment changes the charge properties at or near the smectite particle surface.

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